Hogg S. Essential microbiology

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58 CELL STRUCTURE AND ORGANISATION

Endospores

Endospores of pathogens

such as Clostridium bo-

tulinum can resist boiling

for several hours. It is this

resistance that makes

it necessary to auto-

clave at 121

◦

Cinorder

to ensure complete

sterility.

Certain bacteria such as Bacillus and Clostridium pro-

duce endospores. They are dormant forms of the cell

that are highly resistant to extremes of temperature, pH

and other environmental factors, and germinate into new

bacterial cells when conditions become more favourable.

The spore’s resistance is due to the thick coat that sur-

rounds it.

The plasma membrane

The cytoplasm and its contents are surrounded by a plasma membrane, which can

be thought of as a bilayer of phospholipid arranged like a sandwich, together with

associated proteins (Figure 3.5). The function of the plasma membrane is to keep the

contents in, while at the same time allowing the selective passage of certain substances

in and out of the cell (it is a semipermeable membrane).

Phospholipids comprise a compact, hydrophilic (= water-loving) head and a long

hydrophobic tail region (Figure 2.27); this results in a highly ordered structure when

the membrane is surrounded by water. The tails ‘hide’ from the water to form the inside

of the membrane, while the heads project outwards. Also included in the membrane

are a variety of proteins; these may pass right through the bilayer or be associated with

the inner (cytoplasmic) or outer surface only. These proteins may play structural or

functional roles in the life of the cell. Many enzymes associated with the metabolism

of nutrients and the production of energy are associated with the plasma membrane in

procaryotes. As we will see later in this chapter, this is fundamentally different from

Figure 3.5 The plasma membrane. Phospholipid molecules form a bilayer, with the hy-

drophobic hydrocarbon chains pointing in towards each other, leaving the hydrophilic phos-

phate groups to face outwards. Proteins embedded in the membrane are known as integral

proteins, and may pass part of the way or all of the way through the phospholipid bilayer.

The amino acid composition of such proteins reﬂects their location; the part actually embed-

ded among the lipid component of the membrane comprises non-polar (hydrophobic) amino

acids, while polar ones are found in the aqueous environment at either side. Singleton, P:

Bacteria in Biology, Biotechnology and Medicine, 5th edn, John Wiley & Sons, 1999. Re-

produced by permission of the publishers

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THE PROCARYOTIC CELL 59

eucaryotic cells, where these reactions are carried out on specialised internal organelles.

Proteins involved in the active transport of nutrients (see Chapter 4) are also to be found

associated with the plasma membrane. The model of membrane structure as depicted in

Figure 3.5 must not be thought of as static; in the widely accepted ﬂuid mosaic model,

the lipid is seen as a ﬂuid state, in which proteins ﬂoat around, rather like icebergs in

an ocean.

The majority of bacterial membranes do not contain sterols (c.f. eucaryotes: see

below), however many do contain molecules called hopanoids that are derive from

the same precursors. Like sterols, they are thought to assist in maintaining membrane

stability. A comparison of the lipid components of plasma membranes reveals a distinct

difference between members of the Archaea and the Bacteria.

The bacterial cell wall

Bacteria have a thick, rigid cell wall, which maintains the integrity of the cell, and

determines its characteristic shape. Since the cytoplasm of bacteria contains high

A protoplast is a cell that

has had its cell wall re-

moved.

concentrations of dissolved substances, they generally

live in a hypotonic environment (i.e. one that is more di-

lute than their own cytoplasm). There is therefore a nat-

ural tendency for water to ﬂow into the cell, and without

the cell wall the cell would ﬁll and burst (you can demon-

strate this by using enzymes to strip off the cell wall, leaving the naked protoplast).

Proteases are enzymes

that digest proteins.

The major component of the cell wall, which is re-

sponsible for its rigidity, is a substance unique to bacte-

ria, called peptidoglycan (murein). This is a high molec-

ular weight polymer whose basic subunit is made up of

three parts: N-acetylglucosamine, N-acetylmuramic acid and a short peptide chain (Fig-

ure 3.6). The latter comprises the amino acids l-alanine, d-alanine, d-glutamic acid and

either l-lysine or diaminopimelic acid (DAP). DAP is a rare amino acid, only found in

the cell walls of procaryotes. Note that some of the amino acids of peptidoglycan are

found in the d-conﬁguration. This is contrary to the situation in proteins, as you may

recall from Chapter 2, and confers protection against proteases speciﬁcally directed

against l-amino acids.

Precursor molecules for peptidoglycan are synthesised inside the cell, and transported

across the plasma membrane by a carrier called bactoprenol phosphate before being in-

corporated into the cell wall structure. Enzymes called transpeptidases then covalently

bond the tetrapeptide chains to one another, giving rise to a complex network (Figure

3.7); it is this cross-linking that gives the wall its mechanical strength. A number of

antimicrobial agents exert their effect by inhibiting cell wall synthesis; β-lactam an-

tibiotics such as penicillin inhibit the transpeptidases, thereby weakening the cell wall,

whilst bacitracin prevents transport of peptidoglycan precursors out of the cell. The

action of antibiotics will be discussed further in Chapter 14. Although all bacteria (with

a few exceptions) have a cell wall containing peptidoglycan, there are two distinct struc-

tural types. These are known as Gram-positive and Gram-negative. The names derive

from the Danish scientist Christian Gram, who, in the 1880s developed a rapid staining

technique that could differentiate bacteria as belonging to one of two basic types (see

Box 1.2). Although the usefulness of the Gram stain was recognised for many years, it

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60 CELL STRUCTURE AND ORGANISATION

HOH

COOH

HCH

H (CH

)

CH COOH

HCH

C O

-acetylmuramic

acid residue

-acetylglucosamine

residue

Simplified depiction

–Alanine

–Glutamic acid

meso

–Diaminopimelic acid

–AlanineD

NAM NAG

Figure 3.6 Peptidoglycan structure. Peptidoglycan is a polymer made up of alternating

molecules of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). A short pep-

tide chain is linked to the NAM residues (see text for details). This is important in the

cross-linking of the straight chain polymers to form a rigid network (Figure 3.6). The com-

position of E. coli peptidoglycan is shown; the peptide chain may contain different amino

acids in other bacteria. Partly from Hardy, SP: Human Microbiology, Taylor and Francis,

2002. Reproduced by permission of Thomson Publishing Services

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THE PROCARYOTIC CELL 61

Figure 3.7 Cross-linking of peptidoglycan chains in E. coli. (a) The d-alanine on the short

peptide chain attached to the N-acetylmuramic acid cross-links to a diaminopimelic acid

residue on another chain. In other bacteria, the precise nature of the cross-linking may differ.

From Hardy, SP: Human Microbiology, Taylor and Francis, 2002. Reproduced by permis-

sion of Thomson Publishing Services. (b) Further cross-linking produces a rigid network

of peptidoglycan. The antibiotic penicillin acts by inhibiting the transpeptidase enzymes

responsible for the cross-linking reaction (see Chapter 15)

was only with the age of electron microscopy that the underlying molecular basis of the

test could be explained, in terms of cell wall structure.

Gram-positive cell walls are relatively simple in structure, comprising several layers

of peptidoglycan connected to each other by cross-linkages to form a strong, rigid

scaffolding. In addition, they contain acidic polysaccharides called teichoic acids; these

contain phosphate groups that impart an overall negative charge to the cell surface. A

diagram of the gram-positive cell wall is shown in Figure 3.8.

Gram-negative cells have a much thinner layer of peptidoglycan, making the wall

less sturdy, however the structure is made more complex by the presence of a

layer of lipoprotein, polysaccharide and phospholipid known as the outer membrane

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62 CELL STRUCTURE AND ORGANISATION

Figure 3.8 The Gram-positive cell wall. Peptidoglycan is many layers thick in the Gram-

positive cell wall and may account for 30–70% of its dry weight. Teichoic acids are neg-

atively charged polysaccharides; they are polymers of ribitol phosphate and cross-link to

peptidoglycan. Lipoteichoic acids are teichoic acids found in association with glycolipids.

From Henderson, B, Wilson, M, McNab, R & Lax, AJ: Cellular Microbiology: Bacteria-

Host Interactions in Health and Disease, John Wiley & Sons Inc., 1999. Reproduced by

permission of the publishers

(Figure 3.9). This misleading name derives from the fact that it superﬁcially resembles

the bilayer of the plasma membrane; however, instead of two layers of phospholipid, it

has only one, the outer layer being made up of lipopolysaccharide. This has three parts:

lipid A, core polysaccharide and an O-speciﬁc side chain. The lipid A component may

act as an endotoxin, which, if released into the bloodstream, can lead to serious con-

ditions such as fever and toxic shock. The O-speciﬁc antigens are carbohydrate chains

whose composition often varies between strains of the same species. Serological meth-

ods can distinguish between these, a valuable tool in the investigation, for example, of

the origin of an outbreak of an infectious disease. Proteins incorporated into the outer

membrane and penetrating its entire thickness form channels that allow the passage of

water and small molecules to enter the cell. Unlike the plasma membrane, the outer

membrane plays no part in cellular respiration.

Box 3.1 Mesosomes − the structures that never were?

When looked at under the electron microscope, Gram-positive bacteria often con-

tained localised in-foldings of the plasma membrane. These were given the name

mesosomes, and were thought by some to act as attachment points for DNA dur-

ing cell division, or to play a role in the formation of cross-walls. Others thought

they were nothing more than artefacts produced by the rather elaborate sample

preparation procedures necessary for electron microscopy. Nowadays, most micro-

biologists support the latter view.

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THE PROCARYOTIC CELL 63

Figure 3.9 The Gram-negative cell wall. Note the thinner layer of peptidoglycan compared

to the gram-positive cell wall (Figure 3.8). It accounts for <10% of the dry weight. Beyond

this lies the outer membrane, with its high lipopolysaccharide content. Channels made of

porins allow the passage of certain solutes into the cell. From Henderson, B, Wilson, M,

McNab, R & Lax, AJ: Cellular Microbiology: Bacteria-Host Interactions in Health and

Disease, John Wiley & Sons Inc., 1999. Reproduced by permission of the publishers

Members of the Archaea have a cell wall chemistry quite different to that described

above (see Chapter 7). Instead of being based on peptidoglycan, they have other complex

polysaccharides, although a distinction between gram-positive and gram-negative types

still occurs.

Beyond the cell wall

A number of structural features are to be found on the outer surface of the cell wall; these

are mainly involved either with locomotion of the cell or its attachment to a suitable

surface.

Perhaps the most obvious extracellular structures are ﬂagella (sing: ﬂagellum), thin

hair-like structures often much longer than the cell itself, and used for locomotion in

many bacteria. There may be a single ﬂagellum, one at each end, or many, depending

on the bacteria concerned (Figure 3.10). Each ﬂagellum is a hollow but rigid cylindri-

cal ﬁlament made of the protein ﬂagellin, attached via a hook to a basal body, which

secures it to the cell wall and plasma membrane (Figure 3.11). The basal body com-

prises a series of rings, and is more complex in Gram-negative than Gram-positive

bacteria. Rotation of the ﬂagellum is an energy-dependent process driven by the basal

body, and the direction of rotation determines the nature of the resulting cellular move-

ment. Clockwise rotation of a single ﬂagellum results in a directionless ‘tumbling’,

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64 CELL STRUCTURE AND ORGANISATION

(a) Polar, monotrichous

(b) Polar, amphitrichous

(d) Peritrichous

Figure 3.10 Flagella may be situated at one end (a & b), at both ends (c) or all over the

cell surface (d)

but if it rotates anticlockwise, the bacterium will ‘run’ in a straight line (Figure

3.12). Likewise, anticlockwise rotation causes bunched ﬂagella to ‘run’ by winding

around each other and acting as a single structure, whilst spinning in the opposite

direction gives rise to multiple independent rotations and tumbling results once more.

Pili (sing: pilus) are structures that superﬁcially resemble short ﬂagella. They differ

from ﬂagella, however, in that they do not penetrate to the plasma membrane, and

they are not associated with motility. Their function, rather, is to anchor the bacterium

to an appropriate surface. Pathogenic (disease-causing) bacteria have proteins called

adhesins on their pili, which adhere to speciﬁc receptors on host tissues. Attachment

pili are sometimes called ﬁmbriae, to distinguish them from another distinct type of

pilus, the sex pilus, which as its name suggests, is involved in the transfer of genetic

information by conjugation. This is discussed in more detail in Chapter 12.

Outside the cell wall, most bacteria have a polysaccharide layer called a glycocalyx.

This may be a diffuse and loosely bound slime layer or a better deﬁned, and generally

thicker capsule. The slime layer helps protect against desiccation, and is instrumental in

the attachment of certain bacteria to a substratum (the bacteria that stick to your teeth

are a good example of this). Capsules offer protection to certain pathogenic bacteria

against the phagocytic cells of the immune system. Both capsules and slime layers are

key components of bioﬁlms, which form at liquid/solid interfaces, and can be highly

signiﬁcant in such varied settings as wastewater treatment systems, indwelling catheters

and the inside of your mouth!

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THE EUCARYOTIC CELL 65

flagellum

outer membrane

inner membrane

switch complex

cell wall

Figure 3.11 Bacterial ﬂagella are anchored in the cell wall and plasma membrane. The

ﬁlament of the ﬂagellum is anchored by a basal body. In Gram-positive organisms, this

comprises two rings inserted in the plasma membrane. In Gram-negative organisms (as

shown), there are additional rings associated with the outer membrane and the peptidoglycan

layer. Some modern interpretations of ﬂagellar structure view the M and S rings as a single

structure. Energy for rotation of the ﬂagellum is derived from the proton motive force

generated by the movement of protons across a membrane (see Chapter 6). From Bolsover,

SR, Hyams, JS, Jones, S, Shepherd, EA & White, HA: From Genes to Cells, John Wiley &

Sons, 1997. Reproduced by permission of the publishers

The eucaryotic cell

We have already seen that eucaryotic cells are, for the most part, larger and much more

complex than procaryotes, containing a range of specialised subcellular organelles (Fig-

ure 3.13). Within the microbial world, the major groups of eucaryotes are the fungi and

the protists (protozoans and algae); all of these groups have single-celled representatives,

and there are multicellular forms in the algae and fungi.

Our survey of eucaryotic cell structure begins once more with the genetic material,

and works outwards. However, since many internal structures in eucaryotes are en-

closed in a membrane, it is appropriate to preface our description by brieﬂy considering

eucaryotic membranes. These are, in fact, very similar to the ﬂuid mosaic structure we

described earlier in this chapter, as depicted in Figure 3.5. The main difference is that

eucaryotic membranes contain lipids called sterols, which enhances their rigidity. We

shall consider the signiﬁcance of this when we discuss the plasma membrane of eucary-

otes below. Cholesterol, which we usually hear about in a very negative context, is a

very important sterol found in the membranes of many eucaryotes.

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RUN

TUMBLE

Figure 3.12 Running and tumbling. Anticlockwise rotation of the ﬂagellum gives rise to

‘running’ in a set direction. Reversing the direction of rotation causes ‘tumbling’, and allows

the bacterial cell to change direction

Figure 3.13 This example of eucaryotic cell structure shows a plant cell. Other eucaryotic

cells may differ with respect to the cell wall and the possession of choroplasts. Note the

much more elaborate internal structure compared to a typical procaryotic cell (Figure 3.3), in

particular the presence of membrane-bounded organelles such as mitochondria, chloroplasts,

endoplasmic reticulum and a true nucleus. From Nicklin, J, Graeme-Cook, K & Killington,

R: Instant Notes in Microbiology, 2nd edn, Bios Scientiﬁc Publishers, 2002. Reproduced by

permission of Thomson Publishing Services

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THE EUCARYOTIC CELL 67

The nucleus

The principal difference between procaryotic and eucaryotic cells, and the one that

gives the two forms their names, lies in the accommodation of their genetic material.

Eucaryotic cells have a true nucleus, surrounded by a nuclear membrane. This is in fact a

double membrane; it contains pores, through which messenger RNA leaves the nucleus

on its way to the ribosomes during protein synthesis (see Chapter 11).

A cell containing only

one copy of each chro-

mosome is said to be

haploid. The term is also

applied to organisms

made up of such cells

The haploid state is of-

ten denoted as N. (c.f.

diploid (2N): contain-

ing two copies of each

chromosome)

The organisation of genetic material in eucaryotes is

very different from that in procaryotes. Instead of ex-

isting as a single closed loop, the DNA of eucaryotes is

organised into one or more pairs of chromosomes. The

fact that they occur in pairs highlights another important

difference from procaryotes: eucaryotes are genetically

diploid in at least some part of their life cycle, while pro-

caryotes are haploid. The DNA of eucaryotic chromo-

somes is linear in the sense that it has free ends; however,

because there is so much of it, it is highly condensed and

wound around proteins called histones. These carry a

strong positive charge and associate with the negatively

charged phosphate groups on the DNA.

As well as the chromosomes, the nucleus also contains

the nucleolus, a discrete structure rich in RNA, where ri-

bosomes are assembled. The ribosomes themselves have

the same function as their procaryotic counterparts;

the differences in size have already been discussed (see

A histone is a basic pro-

tein found associated

with DNA in eucaryotic

chromosomes.

Table 3.3). They may be found free in the cytoplasm or

associated with the endoplasmic reticulum (see below),

depending on the type of protein they synthesise.

Endoplasmic reticulum

Running throughout the cell and taking up much of its volume, the endoplasmic retic-

ulum (ER) is a complex membrane system of tubes and ﬂattened sacs. The presence of

numerous ribosomes on their surface gives those parts of the ER involved in protein syn-

thesis a granular appearance when seen under the electron microscope, giving rise to the

name rough ER. Areas of the ER not associated with ribosomes are known as smooth

ER; this is where the synthesis of membrane lipids takes place. The ER also serves as a

communications network, allowing the transport of materials between different parts

of the cell.

Golgi apparatus

The Golgi apparatus is another membranous organelle, comprising a set of ﬂattened

vesicles, usually arranged in a stack called a dictyosome. The function of the Golgi